(1) Field of the Invention
The present invention relates to an image forming apparatus that forms an image on recording sheets, and in particular to a drying technique to dry a developer such as toner and ink that constitute the image formed on the recording sheets.
(2) Description of the Related Art
An image forming apparatus (e.g., a copying machine or a printer) that forms a toner image by an electrophotographic method normally transfers the toner image formed on a photosensitive drum onto a recording sheet, and then fixes the toner image by a fixing device. The fixing device heats the toner image on the recording sheet and applies pressure on the recording sheet to fix the toner image. The recording sheet on which the toner image has been fixed is ejected on an ejected-sheet tray.
The recording sheet is placed onto the ejected-sheet tray with a side on which the toner image has been formed facing down on the ejected-sheet tray. When toner images are successively formed on a plurality of recording sheets, the recording sheets are sequentially placed on the ejected-sheet tray with the toner images facing down.
The toner image on each recording sheet is fused at the fixing device, and then dried while the recording sheet is transferred from the fixing device and ejected onto the ejected-sheet tray. However, if the toner image on the recording sheet ejected onto the ejected-sheet tray is not completely dried, the toner image on the recording sheet placed on the ejected-sheet tray might adhere to another recording sheet placed thereunder. A problem that recording sheets adhere to each other with toner is normally called “fixing tacking”.
Such fixing tacking is more likely to occur, for example, when a temperature of a recording sheet after fixation is high, when an ambient temperature of the fixing device is high, and when an ambient temperature around the image forming apparatus is high. This is because a drying speed of toner becomes slow.
Also, when duplex printing for forming toner images on both sides of a recording sheet is performed, fixing tacking is more likely to occur since toner images exist on opposite sides facing each other of recording sheets layered on the ejected-sheet tray. Furthermore, when a basis weight of a recording sheet is large, fixing tacking is more likely to occur since toner on an upper recording sheet is pressed with a high pressure against a recording sheet that is positioned under the upper recording sheet with its own weight.
Also, when a coverage ratio that is a ratio of a toner image to a surface of a recording sheet is high, an amount of toner is large and therefore fixing tacking is more likely to occur.
Moreover, when a speed (system speed) of transportation of recording sheets becomes high in successively performing image formation, intervals of transportation of recording sheets ejected onto the ejected-sheet tray become short. Therefore, a recording sheet might be placed on a preceding recording sheet before a toner image on the recording sheet has not completely dried. As a result, fixing tacking is more likely to occur.
Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 2004-291654) discloses a structure for drying ink in an inkjet recording apparatus that can perform duplex printing, according to which a rear end portion of a recording sheet is sandwiched between a switch back roller and a switch back runner while ink is drying.
Also, Patent Literature 2 (Japanese Unexamined Patent Application Publication No. 2006-103849) discloses an ink jet image forming apparatus that forms an image on a recording medium by an ink jet printing system and then dries ink by a blast fan while temporarily holding the recording medium with use of a sheet ejecting roller.
According to the structure disclosed in Patent Literature 1, in duplex printing, printing on one side of a recording sheet must be performed after ink on the other side of the recording sheet dries. This results in reduction of printing efficiency.
Also, according to the structure disclosed in Patent Literature 2, printing efficiency decreases since printing cannot be performed while the sheet ejecting roller is holding the recording medium and the blast fan is drying the recording medium. Moreover, if an amount of air moved by the blast fan is increased so as to dry ink faster, consumed electric power increases, and at the same time, a loud noise might occur.
Note that in order to prevent reduction of printing efficiency, Patent Literature 2 discloses a structure according to which two or more sheet ejecting paths are provided and a recording medium that does not need to have ink dried by being held by the sheet ejecting roller overtakes a preceding recording medium and is ejected onto the ejected-sheet tray. However, with such a structure, an order of recording mediums ejected onto the ejected-sheet tray changes, and accordingly a user has to rearrange the order of the ejected recording mediums.